The present invention relates to an acoustic method for estimating the mechanical properties of a material and an apparatus therefor. In particular, the invention provides a new method and apparatus for estimating the acoustic properties of set cements used in an oil well or the like.
After drilling an oil well or the like, the annular space surrounding the casing is generally cemented. Such an operation is intended to consolidate the well, to protect the casing and also, essentially, to isolate geological layers so as to prevent fluid exchange between the various formation layers, where such exchange is made possible by the path formed by the drilled hole. The cementing operation is also intended to prevent gas from rising via the annular space and to limit the ingress of water into the production well. Good isolation is thus the primary objective of the majority of cementing operations carried out in oil wells or the like.
Consequently, the selection of a cement formulation is a critical issue in such cementing operations. The appropriate cement formulation helps to achieve a durable zonal isolation, which in turn ensures a stable and productive well without requiring costly repair. Important parameters in assessing whether a cement formulation will be optimal for a particular well environment are the mechanical properties of the cement after it sets inside the annular region between casing and formation. Compressive and shear strengths constitute two important cement mechanical properties that can be related to the mechanical integrity of a cement sheath. These mechanical properties are related to the linear elastic parameters namely: Young's modulus, shear modulus, and Poisson's ratio. It is well known that these properties can be ascertained from knowledge of the cement density and the velocities of propagation of the compressional and shear acoustic waves inside said cement. Thus, ultrasonic measurements can be used: these non invasive measurements are particularly interesting as opposed to other well known mechanical techniques whereby samples are stressed to a failure stage to determine their compressive or shear strength.
Acoustic tools are usually used to perform these acoustic measurements. These tools are lowered inside a well to evaluate the cement integrity through the casing. However, interpretation of the acquired data is particularly difficult, especially due to the various compositions of the cements and data derivation induced by—among other—electronics inside said acoustic tools. Consequently, several mathematical models have already been developed to simulate the measurements, those models being very helpful to anticipate the performance of the evaluation tools as well as to interpret the tool data. However, those models necessitate the knowledge of both the velocities and attenuations of acoustic waves that propagate inside the cement. Thus, providing accurate values to those models is a very important aspect in interpreting the measurements of the different acoustic tools.
U.S. Pat. No. 4,259,868, to Rao et al, describes a device that measures compressive ultrasonic waves transit time through cement samples inserted in an auto-clave held at constant temperature and pressure. The transit time information is converted to compressive strength on the basis of established relationships between the two quantities. These relationships are obtained empirically from carrying out ultrasonic transit time measurements and mechanical compressive strength measurements on many samples. As far as the transit time of compressive ultrasonic waves decreases along the setting of the cement, the device, performed on a cement slurry, provides the cement set time, or the time it takes for the slurry to gain a minimal compressive strength, and the time for development of full compressive strength. These parameters are used as inputs for efficient scheduling and carrying out of a well cementing job. However, the reliability and accuracy of this device to provide cement compressive strength relies on the accuracy of the correlation relationships that relate compressive strength to compressional wave transit time.
An other patent U.S. Pat. No. 5,741,971 to Lacy also provides the characteristics of cement slurries measured acoustically in an auto-clave. These characteristics include dynamic Young's modulus, density, static viscosity, compressive strength and expansion or contraction of set cement. Young's modulus measurement is based on an estimation of a compressional wave transit time through a cement sample. The estimation of Young's modulus from the transit time information is based on what is believed to be an empirical relation between the two quantities. Furthermore, it is speculated that the derivation of this empirical relation is based on calibration measurements for cements with well known or independently measured Young's moduli. This patent also describes an additional empirical relation to relate the estimated Young's modulus to the unconfined compressive strength of the tested cement. This relation is derived using best fit approaches applied to data relating the two quantities where the compressive strength is measured independently using a conventional measurement.
All this embodiments of ultrasonic measurements are geared towards propagating and detecting compressional waves only. However, since shear waves only propagate through solids media, detecting shear waves in the slurry as a function of time indicates when the transition liquid to solid/gel occurs, which is an important input in determining the mechanical properties of a cement. In the U.S. Pat. No. 5,412,990, granted to D'Angelo et al, the onset of shear waves transmitted through a cement slurry is monitored and directly related to the cement thickening time. Nevertheless, as this method does not take compressional waves into account, it suffers from being inaccurate.
WO 00 34769 discloses a method for estimating the time varying properties of cement involving the propagation of acoustic signals through a sample and measuring signal corresponding thereto to determine the properties of the sample.
U.S. Pat. No. 5,001,676 describes an acoustic well logging tool that has a transducer offset from the borehole wall such that signal emitted thereby strike the borehole wall or casing at an incident angle that directs them away from the transducer.
GB 2,293,653 describes an acoustic method for non-destructive determination of porosity of a rock sample based on the travel time of shear and compressional waves from acoustic pulse between spaced transmitters and receivers.
RU 2,006,883 describes techniques for estimating the quality of cementing in a borehole using measurements of reflected acoustic waves in specified frequency ranges.